Honestly I have no idea if this is close to being correct but I feel like grenades are similar enough to dynamite which is (I think-again no actual clue) still used for mining and construction. It’d be more problematic to delineate one as a construction tool and one as a death tool so they keep them all under the same umbrella. Again, this comment is 1000% unfounded guessing.
Edit: This I do know, federally legal does not mean legal for everyone! There is a similar procedure to follow for purchasing a silencer and I know a ton of people who have them that 50 miles south accross the PA or OH state line but there is absolutely no legal way or work around for me to legally own a silencer living in NYS.